This article originally appeared on Tonic.
In Saskatchewan, British Columbia, a woman was murdered. The police strongly suspected that her husband killed her three weeks before they found her body. But his friends claimed they saw his wife, alive and well, just one week earlier—when her husband had a solid alibi.
"The question was: Had she died three weeks before, when the police believed she had and when her husband had no alibi, or had she died a week before, when he was fully alibied?" Gail Anderson says. "I didn't know any of this of course, I was just looking at the insects."
Anderson was the forensic entomologist called in to help, an expert on insects and the specific ways they colonize decomposing flesh, be it animal or human. There are around 100 members of the North American Forensic Entomology Association. While most spend their time in academic positions and conducting research, they are often called upon to consult in criminal cases, and provide evidence for time of death and other crucial details of a homicide. Unlike people, Anderson says, insects don't lie.
"There are lots of cases where we have eye witnesses saying that they'd seen the death take place on one date, and other eyewitnesses say No, I saw her alive shopping at the mall," Anderson says. "The jury are then asked who do they believe, who do they trust. Our entire jury system is very subjective. You're asking a person if they can tell whether somebody is lying. We can't. People lie to us all the time, and we believe them. For us, it's not subjective. It's purely objective evidence. I can't tell you who's telling the truth, but I can tell you about flies."
On the woman's body from Saskatchewan, she found insects that were at least three weeks old, based on the stage of development they had reached. She gave her analysis to the police, and they re-questioned the husband's friends. This time, when they said they had seen her a week before, the police had a rebuttal.
"They slammed my report down on the table, and said, 'We've got scientific evidence to prove you're lying to us, and that she was dead three weeks ago'," Anderson says. "And they didn't even look at my report, but once they were told there was forensic evidence against them, they rolled and said, 'Yea, ok, maybe it was three weeks ago, can't really remember.' Once the suspect heard his witnesses had rolled, he confessed to a lesser charge, second degree murder– it never even went to court."
"I can't tell you who's telling the truth, but I can tell you about flies."
Blow flies and flesh flies are both attracted to the odors of open wounds or a dead body. Blow flies, the blue and green buzzy metallic flies, will arrive first, and the flesh flies won't be far behind. Once they start using the body as a food source, they will lay eggs or larvae directly into it, which hatch in about 24 hours into first-stage maggots.
Blow flies are small, their eggs are about the size of a grain of rice or smaller. When their eggs hatch, the maggots crawl into natural body openings, and they wouldn't be obvious to a casual on-looker. But these tiny maggots eat—a lot. They feed 24 hours a day, and increase their body size 300-400 times by the time they reach the last stage of development, the pupil stage (like the cocoon stage for butterflies). Flesh flies are much bigger, and their maggots follow suit. Those maggots will be about one inch long, easily seen wriggling on and around the body. The maggots will eventually pupate and transform into adult flies, and repeat until the body is completely decomposed.
"Most people tend to be a little bit frightened, scared, repulsed by insects, and so when you put it in the context that insects are feeding on a human body and that you can use that information, they find the people that do that, well, fascinating, but also incredibly odd," says Dave Rivers, a forensic entomologist at Loyola University Maryland, who teaches the only class on the subject in his state.
Though it's only been used in our judicial system for the past 20 years, forensic entomology is not a new field. It was a documented technique as long ago as the 10th century. One example, recorded in a Chinese training manual on investigating death from 1235, explains how blow flies on a sickle prompted a man's confession that he killed another Chinese farm worker with that sickle.
From the 13th to the 19th century, biologists recorded the stages of development flies went through, classified of the different kinds of eggs and development times. That information came in handy in several cases, like in France in 1850, when a murdered newborn baby was found encased in a chimney. Marcel Bergeret, a doctor and naturalist, found flesh fly larvae and moths, and determined that the body had been sealed in the chimney in 1848, and the moths had arrived in 1849. This timeline exonerated the people who were currently living in the house, and implicated the previous occupants from 1848.
In 1935, female body parts were found in a river near Edinburgh, Scotland. After being identified, investigators found blowfly larvae in the third stage of development on their bodies, implying that the eggs had been laid before the bodies were thrown into the river. That time estimate, along with other evidence, led to the conviction of one of the women's husbands.
Whether a body is encased in a chimney or thrown in a river, insects will always manage to find it. Today, even inside luxury cars, or shut inside apartments, no body is immune. "Insects are ferocious at searching for food," Rivers says. "When someone commits a heinous crime, and they try to cover their tracks by leaving that body indoors, or burying that body, putting it in a container, wrapping it in something, throwing that body into water, that doesn't prevent the insects from eventually getting to it. It may delay them getting there. But if the conditions are right, they will get there."
Investigators found that eggs had been laid before the bodies were thrown into the river. That led to the husband's conviction.
Since maggots are born directly onto human remains and will use it to grow, they offer a unique clock to gauge the body with: "The fly's development is now directly tied to the corpse," Rivers says. This helps answer the most commonly asked question of a forensic entomologist: What's the minimum time a person has been dead?
The most basic way to answer that is to identify the species of insect and see how far through its life it has gotten. Development depends on many factors like temperature and climate, and is different for each species, says Eric Benbow, a forensic entomologist at Michigan State University. Benbow did his PhD studying the life cycle of insects in Hawaiian streams, and how humidity, moisture, and temperature affect how fast they grow.
When a forensic entomologist arrives at a crime scene, they'll record the temperature, see how it's been changing over the last month, and calculate the average temperature every day. Cross-comparing growth-rate tables of different species with the temperature allows them an accurate read of exactly how long the bugs have been on the body. They take a sample of the larvae back to the lab, freeze half to keep a record of exactly what stage they're in, and rear the rest to adulthood to get an ID on the insect species.
But there's rarely just one insect on a body. As the body decomposes it goes through rapid biological, chemical changes. Each one of those changes is attractive to a different group of insects. Some insects like the body when it's fresh, others like it when it's a bit older, a bit dryer. Others prefer when it's even more dry, just skin and bones.
Forensic entomologists aren't just calculating the age of one type of fly, but all the insects that are present, and estimating their time of arrival. "It also depends on if the body is here, where you are, in Frankfurt or in Australia," Anderson says. "It's going to be different if it's in sun or shade. Spring, summer, fall, all of these kinds of things will affect what insects will colonize and when they colonize."
It can become complicated very quickly, and you can see why it takes an expert to find and wrangle the evidence left over from this morbid menagerie. Sometimes it's the evidence from the insects already come and gone—which can be as small as a leftover pupae case—that is most helpful.
Sometimes insects will entirely change the course of an investigation. In one of Rivers' cases, a body was found in a wooded area in Minneapolis. There were flies on the body, and they had reached a developmental stage that attracted parasitic wasps. When he looked closer at the wasps, he found their stage of development didn't match the season or temperature. He concluded that the body had been in another location, possibly indoors, or from another region altogether. Then, when analyzing the fly species, he discovered they were more typical of the Southwestern United States than Minnesota.
When the details of the case came out, his report helped provide scientific evidence against the killer, who had committed the murder in Texas, put the body in the trunk of his car, and drove it to Minneapolis to dump it. If a body has been moved, the bugs will give it away. "It doesn't have to be that extreme," Rivers says. "I'm located in Maryland, and as you travel east to west in the state, the fly fauna change fairly dramatically."
Since the maggots are feeding on the body, they can also hold valuable information, like the victim's DNA, in a case where the body has been destroyed or is missing. Rivers says that maggots can be used to test if a victim had been taking drugs, or poisoned. In Baltimore, the police found a body so severely decomposed that the couldn't get any information from it. Testing the maggots on the body, they could determine that the victim had been on strong anti-psychotic medication, and could begin to piece together what had happened.
Maggots can also consume, and be tested for, gunshot residue—allowing investigators to prove someone had been shot if a body is too decomposed to see any wounds. Some experiments are now finding that you can identify bomb residue within the maggots as well, and can even differentiate among types of bombs—and lead investigators to the organizations responsible for them. Bed bugs that fed on a suspect could prove, via DNA from their blood, that a suspect was in a specific location.
Soon, forensic entomologists may be able to determine a maggot's age even more precisely, using gene expression. Certain genes in different species are turned on or off, under specific conditions, and Benbow says they're hoping to be able to look at those genes to know exactly how old they are. When determining if a body has been moved, forensic entomologists are starting to use population genetics to determine—even within the same species—if a bug came from somewhere else.
Benbow's work is now taking a closer look at the microbes that the insects carry. When a body decomposes, it holds very different microbes in the first 24 hours compared to the next 72, he says. Understanding the differences in microbial communities and their relationship with the insects feeding on them can offer even more precision and information about time of death and colonization. "All of this doesn't solve the crimes, but it allows investigators to ask additional questions they couldn't before," Benbow says.
We can now identify bomb residue within the maggots.
As helpful as insects are, Rivers is dedicating a large part of his current research to understanding the fly's other behaviors, and how they can cloud evidence. "The qualities that make insects useful for trying to estimate the time of colonization—the fact that they feed on the body—is actually considered to be a problem as well," he says.
Flies are messy eaters. Rivers calls them "sloppy." When they feed, they take in as much liquid as possible, but they don't process it right away. It goes into a pouch outside their stomach, called a crop, and they dump enzymes into it to start breaking it down. But they have a tendency to regurgitate that blood and enzyme mixture as a bubble.
"A fly might blow a bubble, and they're mixing it with the enzymes, but then they drop it, and when they do, it looks like blood—because that's what they just fed on," Rivers said. "And it has the appearance of certain kinds of blood stains created by a variety of different mechanisms that would be consistent with blunt force trauma or a gunshot."
The exact shape and size of blood stains is what helps blood-stain experts determine what kind of injury led to death. If investigators tested the fly splatter to see if it was human, the test would come back positive, because it is human blood. Rivers says that even the most advanced tests can't tell if it's human blood from a fly, or directly from a body. The only way experts can tell is to determine based on shape.
When an adult fly first lands in a pool of wet blood, it also walks through it, changing the appearance of the stain. Flies, with their tiny foot prints, add tails and wisps onto the stains, making them look like higher-impact splatters. They can also move blood to areas that have nothing to do with the crime. Since flies are attracted to light sources, they could walk through a blood stain, and then leave footprints or regurgitation on lamps or windows, distorting perceptions about whatever took place.
"The expert makes a subjective determination and says, Based on the appearance and my years of experience, I can tell you that it definitively is fly and not blood," Rivers says. "This is one of those areas that hasn't had the spotlight, but I personally have been trying to address it to say: You cannot give expert testimony that's just based on opinion."
His lab is currently developing an antibody test that will recognize unique proteins in fly spit and feces that aren't present in human blood or any other human body fluids. The hope is that it will allow investigators to definitively tell the difference, and be able to use that evidence in court.
But flies are messy eaters. They can move blood to areas that have nothing to do with the crime.
Michelle Sanford, the only forensic entomologist employed full-time by a medical examiner's office—at the Harris County Institute of Forensic Sciences in Houston—welcomes the idea of a precise test for fly residue. She says that while forensic entomology in an academic setting can focus on hypotheticals and developing newer tools, she cannot dabble in uncertainties in her job.
She's held her position for the past four years. In that time, she's been able to match the blood in ticks found on bodies to travel records, or determine if someone died because of a traumatic injury when a body was too decomposed to tell. Maggots will colonize wounds first, so when she sees a bunch of maggots accumulated on the gut, or any other body part, that are older than the rest—she can be pretty sure there was a trauma there.
In one of her cases, two people were found dead and decomposing in an apartment. One was the caretaker of the other, and they were both elderly and had medical problems. What had taken place? Sanford used the age of the insects, and the identification of species on each body, to find that the caretaker had died first, and then the second had died, probably because of the lack of a caretaker. Sanford could tell, with an accuracy of just three to four days, the difference between time of colonization.
"I think that having a forensic entomologist in the medical examiner's office is really beneficial, and I hope that other offices start making that commitment," she says. Nonetheless, her daily life is quite different from how she imagined when she began her education. Sanford, like most other forensic entomologists, trained as a biologist and entomologist first. Now she is a frequent guest at autopsies, crime scenes, body farms and morgues.
Jeff Tomberlin, from Texas A&M University, probably has one of the highest case loads of any US forensic entomologist—about a dozen per year, totaling around 120 cases. He grew up on a farm in Georgia and helped raise livestock; he says it prepared him for the task.
"I got to see the circle of life," he says, "from animals being born to animals being slaughtered and consumed. In this field, when you say a person dies, or an animal dies, are they really dead? It's still very much alive, just not in the sense that we think of it. And that's what we look at."
What the forensic entomologists are experts in, he says, is the life of death. While no one likes to think of insects feeding on their loved ones, or themselves, it can offer a new perspective on the cycle of not only the insects' lives, but our own.
"It can be jarring as a biologist in the life sciences." Benbow says. "But with every death comes more life. That's a better way to think about it, for sure."